To allow a wide range of motion at the hip, the joint is configured as a ball and socket arrangement. These joints provide for a wide range of motion. In a hip joint, the femoral head is joined to the proximal femur by a neck which is angularly disposed relative to the axis of the femur. The natural socket or acetabulum located in the pelvis, receives the femoral head and cooperates with it to form a joint which permits relative motion. Deterioration of the acetabulum and/or femoral head can be brought about by injury or various progressive diseases such as osteoarthritis. When injury or disease occurs, the damaged component can be replaced or rebuilt using an appropriate prosthetic device. Various techniques have been developed over the years for replacing injured or deteriorated joint components. In the technique known as a "total hip replacement", one method involves cementing or press-fitting a cup into the acetabulum. In this technique, the entire natural femoral head and neck are removed and a femoral prosthetic device comprising an elongated stem, a neck and a bearing head is implanted within the proximal femur.
In any event, acetabular implants are well known in the art for replacing the socket portion of a patient's hip joint. Typical acetabular cup assemblies comprise a metal cup shell which are adapted to be secured within the patient's acetabulum, and an inner liner of plastic material which provides a spherical beating surface for receiving the femoral bearing head portion of the femoral prosthetic device. The metal cup shell may be provided with an external thread arrangement to facilitate anchoring to the pelvic bone or it may be secured by other means such as cement, screws or simply an interference fit. The plastic insert can be secured within the metal cup in a variety of methods which include using retaining rings, press fitting or force fitting the plastic insert into the interior of the metal cup and/or thermally fining the plastic insert into the interior of the metal cup.
A particular problem that can occur with prior art acetabular cup designs is that they can be damaged by the tools used for impacting or driving them into the acetabulum. This is especially true regarding acetabular cup designs having the plastic bearing insert already installed into the metal shell prior to the cup being implanted into the acetabulum.
In partial or total hip replacement surgery utilizing this type of acetabular cup, the surgeon places the impaction tool used for positioning and driving the cup into the acetabulum, onto the metal shell or onto the plastic insert.
In the case where the tool is placed on the metal shell slippage of the impaction tool can occur if the metal shell has no provision for receiving the impaction tool. If the impaction tool slips off the metal shell it can damage the plastic bearing insert. Once the bearing insert is damaged, the surgeon may have to replace it and/or remove and replace the entire cup which is quite costly and time consuming. Moreover, since the impaction tool is not securely coupled to the acetabular cup, it may be difficult to properly orient the acetabular cup in the acetabulum during impaction. This presents a serious problem because in order to realize maximum performance from this surgical procedure, the acetabular cup must be properly positioned in the acetabulum. If the acetabular cup is incorrectly implanted, dislocations of the hip joint, decreased range of motion, and eventual loosening or failure of the prosthetic components can occur.
In the case where the impactor tool is placed onto the plastic insert, the impactor may cause permanent deformation and provide for less than optimum contact with the femoral prosthetic bearing head. This can result in accelerated generation of poly debris due to poor contact and a disturbance of the plastic insert locking mechanism.
The present invention overcomes these problems by providing means for coupling a specially designed cup impactor tool to the acetabular cup.
It is, therefore, an object of the present invention to provide an improved acetabular cup design, method and tool for installing the cup which allows the acetabular cup to be installed in the correct orientation and as an assembled unit without damaging the plastic bearing insert.